L. Miao et al., BUDDING TRANSITIONS OF FLUID-BILAYER VESICLES - THE EFFECT OF AREA-DIFFERENCE ELASTICITY, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 49(6), 1994, pp. 5389-5407
Budding and vesiculation are prominent shape transformations of fluid
lipid-bilayer vesicles. We discuss these transitions within the contex
t of a curvature model which contains two types of bending energy. In
addition to the usual local curvature elasticity kappa, we include the
effect of a relative areal stretching of the two monolayers. This are
a-difference elasticity leads to an effective nonlocal curvature energ
y characterized by another parameter kappaBAR. We argue that the two c
ontributions to the curvature energy are typically comparable in magni
tude. The model interpolates smoothly between the spontaneous-curvatur
e model (kappaBAR = 0) and the bilayer-couple model (kappaBAR --> infi
nity), discussed previously in the literature. Conceptually, this mode
l is not new; however, neither its consequences nor its relation to ex
periment has previously been explored in detail. In particular, buddin
g is discontinuous (first order) for small kappaBAR but changes via a
tricritical point to continuous (second order) for large kappaBAR. The
order of the budding transition depends on both the ratio kappaBAR/ka
ppa (which is a material parameter) and the initial area difference be
tween the inner and outer monolayers (which can be modified by appropr
iate treatment of the vesicle). Estimates suggest that, under typical
laboratory conditions, the budding process should be discontinuous, in
apparent disagreement with some recent experiments. Possible reasons
for this discrepancy are discussed. We propose, in particular, that hy
steretic effects are important and that the observed behavior may refl
ect a spinodal instability.